1. Technical Field
The present invention relates to a technique capable of controlling screen display.
2. Related Art
In the past, a screen display control device having a function of processing image data, which is disclosed in JP-A-2007-264572 (paragraphs 0017 and 0018, FIG. 1, etc.), for example, was known. The screen display control device generally includes an image memory (video RAM) temporarily storing the processed image data and performs screen display by periodically reading the processed image data stored in the image memory. In recent years, however, a screen display apparatus such as a liquid crystal display, a plasma display, or an organic EL display has a capability to realize high definition and high gray scale, thereby increasing a volume of image data to be displayed in the screen display apparatus. Therefore, the image memory storing the increased volume of image data needs to have a large capacity, thereby causing an increase in manufacture cost. As a method of solving this problem, a technique capable of compressing the image data by a known data compression method such as a predictive encoding (DPCM encoding) method of encoding differences of adjacent pixel data and storing the compressed data in the image memory was suggested. In addition, upon performing screen display, original image data is restored by reading the compressed data from the image memory and decompressing the compressed data.
However, for example, when the image data are compressed in a line unit and the compressed data are stored in the image memory, processes of writing and reading the compressed data to and from the image memory in a line unit are performed. Since the processes of writing and reading the compressed data are performed asynchronously, a line of the compressed data to be written to the image memory and a line of the compressed data to be read from the image memory are the same in some cases. When the process of writing the compressed data to the image memory and the process of reading the compressed data from the image memory are performed on the same line, the following problems may occur. That is, the process of writing newly compressed data is performed ahead during the process of reading the compressed data stored in the image memory, or the process of reading the compressed data is performed ahead during the process of writing newly compressed data to the image memory. The image data are compressed using the differences of the adjacent data, as described above. Therefore, when the writing process is performed ahead or the reading process is performed ahead, the image data are not able to be restored after the above problem occurs even in the process of reading the compressed data. In order to solve this problem, there is a known a technique in which the newly compressed data to be written are temporarily maintained in a buffer memory and the newly compressed data maintained in the buffer memory are written to the image memory after the process of reading the compressed data from the image memory is completed, when the process of writing the compressed data to the image memory and the process of reading the compressed data from the image memory are performed on the same line. However, this technique has a problem since efficiency is poor in that the newly compressed data are not able to be written to the image memory until the process of reading the already compressed data from the image memory is completed after maintaining the newly compressed data in the buffer memory. Therefore, an improved technique is necessary.